1. Field of the Invention
The present invention relates generally to a Time Division Multiple Access (TDMA) communications apparatus in which a plurality of time slots are allocated in each frame which is treated as a unit of data exchanged in TDMA communication.
2. Description of the Prior Art
In TDMA communication, it is necessary to synchronize slots between communicating stations. As an example, a global positioning system (GPS) is used as means for providing reference time to be used for synchronizing slots in TDMA communication between multiple stations. When TDMA communication is performed between ships individually equipped with Universal Shipborne Automatic Identification Systems (AISs) of which installation was mandated on large vessels in July 2002, a GPS minute signal (GPS 00-second signal) obtained from a GPS receiver, which is also installed on each ship, is input to the TDMA communications apparatus. The TDMA communications apparatus synchronizes frames every minute using the GPS minute signal to ensure correct synchronization of slots between the ships.
If, however, the TDMA communications apparatus on one ship can not utilize the GPS minute signal due to unavailability of the GPS receiver or a failure of the GPS receiver, for example, the TDMA communications apparatus would loose correct slot synchronization, resulting in an inability to conduct TDMA communication.